Rare earth elements, which have physically peculiar characteristics because of their electron arrangements different from those of normal elements, have been utilized as materials for hydrogen occlusion alloys, secondary cell raw materials, optical glass, powerful rare earth magnets, phosphors, abrasive materials and the like.
In particular, in recent years, since rare earth-nickel-based alloys have high hydrogen occlusion capability, a large amount thereof have been utilized as a raw material for the negative electrodes of nickel hydrogen cells, and the importance for the rare earth elements have become higher than before.
However, at present, since virtually the entire amount of rare earth elements are imported, and since molded products of nickel hydrogen cells or the like have a short service life, there has been a strong demand for establishing a method for effectively recovering expensive rare earth elements from scrap products thereof.
As a method for recovering rare earth elements, a wet method in which those elements are recovered from an aqueous solution formed by dissolving the scraps containing rare earth elements in acid such as a mine acid has been generally known, and the wet method includes a solvent extraction method and a precipitation method.
Specifically, in the case when rare earth elements are mutually separated into respective elements, a precise separation by using the solvent extraction method is carried out (for example, see Patent Document 1). However, since rare earth elements have closely chemically similar characteristics, many steps are required for a device for the solvent extraction. Moreover, since an organic solvent is utilized, a facility in which a fire or the like has to be taken into consideration is required and since COD (Chemical Oxygen Demand) increases in a drain, a reinforcement of a drainage processing is required, and the like, with the result that high costs tend to be required.
On the other hand, in the case of misch metal or the like in which there are a plurality of rare earth elements contained therein and there is no necessity for mutually separating the elements, a precipitation method by which the recovery is carried out at low costs is utilized from the industrial point of view. As the precipitation method, an oxalic acid precipitation method (for example, see Patent Document 2) for recovering as an oxalate acid precipitate and a double sulfate precipitation method (for example, see Patent Document 3) in which a double sulfate of a rare earth sulfate and an alkali sulfate is generated so as to be used for recovery has been known.
However, in the case of the oxalic acid precipitation method, the COD in the drain becomes higher, with the result that high costs tend to be required in the drainage processing in the same manner as in the above-mentioned solvent extraction method.
In contrast, in the double sulfate precipitation method, the COD in the drain is not raised, which is different from the oxalic acid precipitation method. In this double sulfate precipitation method, however, since solubility of a heavy rare earth element becomes extremely high, it is difficult to carry out a sufficient recovery and since the solubility of a light rare earth element is also high, the element remains in the liquid at least in a order of 0.0 n (g/l), resulting in a problem of failing to be completely removed therefrom.